1. Field of the Invention
The invention relates to a data sending/receiving apparatus and, more particularly, to a data sending/receiving apparatus in a system for transmitting image data or the like by a phase modulation method.
2. Related Background Art
There is known a transmission system for variable-length encoding quantized image data in accordance with a transmission rate and phase modulating and transmitting to a transmission medium.
For instance, such a system is used in the image communication through a communication satellite.
FIG. 1 shows a constructional block diagram on the transmission side of a conventional apparatus according to a 4-phase phase modulation method. An image signal is supplied to an input terminal 10. An image coding apparatus 12 performs a variable-length coding of the input image signal in accordance with a transmission rate and, further, adds an error correction code to the coded image signal and generates the resultant image data as two phases of P and Q. In a 4-phase phase modulation apparatus 14, an output of the image coding apparatus 12 is first filtered by roll-off filters 16a and 16b and is 4-phase phase modulated by a modulation circuit 18. The 4-phase phase modulated carrier wave is applied from an output terminal 20 to a transmission medium. A passing frequency band and a roll-off coefficient .alpha. of each of the roll-off filters 16a and 16b have been predetermined in accordance with a transmission rate, a transmission frequency occupied band, an error correcting capability, and the like.
The roll-off filter is a low pass filter having cosine/roll-off characteristics which satisfy Nyquist theory to perfectly eliminate inter-code interference. FIG. 2 shows the roll-off coefficient .alpha. and amplitude frequency characteristics.
Assuming that the roll-off coefficient .alpha. is constant, it is necessary to widen a passing frequency band of the roll-off filter as the transmission rate rises. When a state of a transmission channel is bad, a decoding error rate increases with a decrease in roll-off coefficient .alpha.. On the contrary, when the coefficient .alpha. increases, the decoding error rate decreases. However, the transmission frequency occupied band is widened. That is, there is the following relation among the roll-off coefficient .alpha., transmission rate F, and transmission frequency occupied band B. EQU B=G(.alpha.).multidot.F
where, G(.alpha.) is an increasing function.
When the transmission line state deteriorates in the above transmission system, in order to suppress the increase in decoding error rate, methods of increasing the roll-off coefficient .alpha. in order to raise the error correcting capability by increasing a redundancy and the like are considered. However, those methods are not practical because a change in transmission format or a change in transmission frequency occupied band occur.